I've never ever noticed any grease escaping from any of these spherical joints, in fact I would think that there is normally very little grease in there at initial assembly and the rubber/plastic cover just keeps moisture and grit out of that joint.
So, has someone in the past, maybe even the last
garage you visited not just opened up these TRE covers and tried to inject some normal wheel bearing grease in to try to remove any friction that was causing your steering problems - really just in an attempt to take care of any/all possible
problem areas.
If this is concerning you now as maybe being a possible cause of the light/heavy steering, then I'd suggest that you remove these ends of both TRE from the hub assembly and make sure that they are free to move - but not so free that they could be worn, by doing that you could also check the same issue at the track rod inner end without any more dismantling.
Extra things that you could then do while these two TRE are disconnected is to jack up the front of the car and check how easy it is to turn the front wheels through their normal steering angles now that they are disconnected from the steering rack.
One thing, I would not expect that you can buy new self locking nuts of the same sort as SEAT fitted to these cars, everyone else in the VW Group stopped using that profile of nut and is now using one with a flange at the bottom to spread the loading area - I found that out when replacing the front struts on the late 2009
Ibiza - though I never tried to buy them from SEAT just my local VW Van Centre who could not buy them as they were on back order and remained so for 2 months, so I cancelled that order and bought what VW now fit to 6R Polo.
Your TRE looks very clean/new for that age of car, here in Northern UK, at that age, the TRE are a nasty rusty lump! Once you get these self locking nuts off, you will be able to release the taper bit from the hub assembly by hitting the hub assembly at the area that the TRE taper passes through with TWO hammers at the same time - as "opposite" each other as space allows. If you need to press down on the threaded section, refit the self locking nut upside down to protect the top of the threaded section.